1. Field
Example aspects of the present invention generally relate to systems operating under the ATSC Digital Television Standard (A/53), and more particularly to providing ATSC interoperability by using an external time reference to synchronize the emission of data packets.
2. Related Art
The Digital Television (“DTV”) Standard (or A/53 standard) established by the Advanced Television Systems Committee (“ATSC”) describes the parameters of a system including video/audio encoders, preprocessing/compression parameters, associated multiplexer/transport layer characteristics and normative specifications, and the vestigial-sideband radio-frequency (“VSB RF”) transmission subsystem. Television stations conforming to the standard typically transmit 8-VSB dataframes without regular or known time relationships. This is because the A/53 standard does not specify when a VSB frame should be emitted from a station.
Under the existing ATSC DTV standard, the ATSC symbol clock is not locked to a GPS reference (e.g., 5 or 10 MHz reference signals) and has a tolerance of +/−30 Hz. The VSB dataframes among stations thus have a random frequency and phase relationship causing exciters at different geographic locations to be unsynchronized. As a result, typical ATSC systems do not have an external reference that a remote station can use to lock its data framing.
A modification to the conventional 8-VSB modulation system based on the ATSC transmission standard has been proposed. The modification, referred to as advanced VSB, or A-VSB, builds on the existing ATSC transmission standard to enhance the ability of an ATSC DTV station to transmit signals to new mobile or handheld receivers in dynamic environments while maintaining backward compatibility with legacy ATSC DTV receivers. The proposed A-VSB system also facilitates synchronization of transmitted signals from multiple transmission towers, which improves coverage with higher, more uniform signal strength throughout a service area, even in locations that normally would be shielded by obstacles such as hills or buildings.
U.S. patent application Ser. No. 11/422,791, entitled “APPARATUS, SYSTEMS AND METHODS FOR PROVIDING TIME DIVERSITY FOR MOBILE BROADCAST SERVICES” describes exemplary mechanisms for providing enhancements to ATSC networks using synchronous VSB frame slicing in single transmitter and single frequency networks, and for providing time diversity for mobile broadcasters. FIG. 1 illustrates the construction of what is referred to in that patent application as a “superframe” 106. A superframe 106 is a series of VSB dataframes 104, each of which includes 624 transport stream (TS) packets 102. As shown in FIG. 1, a VSB frame initialization packet (VFIP) can be inserted as the last packet slot of a series of TS packets (102). By the placement of a VFIP in the last packet slot (i.e., packet slot 623), signaling of a VSB frame is made implicit. Upon reception of the VFIP, components such as exciters (not shown) can be signaled to start a new data VSB frame after the last bit of VFIP packet is received. After the first VFIP insertion, additional VFIPs can be inserted at a predetermined periodicity (e.g., approximately once per second).
Data content, such as datacasting data content having news, weather, sports information, and the like, can be inserted into slices within a subset of dataframes (e.g., dataframes 1-3, 4-9, 10-15, 16-20). Slices can be inserted on a dynamic basis since the signaling provides receiving devices with a deterministic mapping as to when the service content will be broadcast. These VSB frames can be multiplexed to generate the superframe 106. RF transmission systems can then broadcast a stream of superframes 106 to mobile or handheld receivers.